First of all, you need to secure your horse to a post or railing by making a good use of cross tie or a single tie. However, make sure that you keep the tie a little loose that does not restrict the horse from moving its head from side to side.
Brush the whole body of your horse but confirm that you select the brush according to the sensitivity of the skin of your horse. If it is more sensitive then use a soft bristled brush and it the skin of your horse is less sensitive then use a medium bristled brush.
Make sure you have brushed the belly, back, legs, neck and head of your horse in order to give it a neat and tidy appearance.
You can also use a spray or any other product over the body of your horse. However make sure you do not apply the product or spray on girth area of saddle of your horse.
Take a soft bristled brush and brush the mouth of your horse gently. You should wipe the eyes of the horse and muzzle by taking a cloth after making it damp. You can also use baby oil on the face of your horse in order to give a shine to its features.
After that, carefully braid the mane of your horse and use elastics or thread to organise it. It will help you in getting good points from the judges who will have a better and clear look on the neck of your horse.
Take a hoof pick and remove any embedded dust or dirt from the hooves of your horse. Use hoof polish and use a brush to apply this polish on the hooves of your horse.